Have you checked the thatch layer? If it is too thick or dense nothing can get through it to the soil/root level. If that's the case core aerate the snot out of it. Though I'd prefer to wait now until fall. Just my 2 cents...
DaleL

What type of grass is it? I have one lawn that I do that was seeded with all Perennial Rye that this "brainiac landscaper put on this new lawn. I am going to sell a slit seeding in the fall to these people for some REAL grass, that will give color and texture. Dirty shame. All that money, and the end result is a light green lawn because of someone else's "skimping".

Joe

Thank you, Dad - for always being the dad that you were. You truly are my hero. You always were.

I have a customer too that just had their lawn hydroseeded with all perennial rye in the fall and i put down a starter fert app in the beginning of the spring and it still looked yellowish after that. I just applied another application of fert(19-2-9). I was wondering if this will help the color of the lawn or is it a hopeless case at this point. Should i resort to a iron product to help the situation??

rye grass will never be a nice dark green, you have to get some blue grass in it. At the very least aerate and overseed with bluegrass heavy in the fall, but be careful with the crabgass control in the spring. What I do is start the season with 19-19-19 on lawns that had been overseeded in the fall and add the pre-m in round 2.